Maraging steels were first discovered circa 25-30 years ago and have witnessed substantial use i sundry and diverse appications. As set forth in U.S. Pat. 4,443,254 ('254), the steels that were of initial commercial significance contained roughly 7-9% cobalt, the cobalt-free versions lacking sufficient toughness for commercial acceptance. The cobalt-free alloy of '254 obviated this drawback and in but a few years has been well received in the marketplace worldwide.
The virtues of '254 notwithstanding, there are applications in which the maraging steel described therein is deemed wanting. Illustrative of this would be appications such as large rocket motor casings where product forms of very substantial thickness are required. As is known, rocket motor casings run 12 -14 feet or more in diameter with a wall thickness of about one-half inch (flange section may run 2-21/2 inchesin thickness). This requires a melt charge of roughly 60,000-65,000 pounds of metal to obtain a forging billet upwards of 40-44 inches thick. Forged rings used in conjunction with such casings also run 12-14 feet in diameter.
Material to be used for rocket motor casings and forged rings should be characterized by a high level of K.sub.IC fracture toughness as well as strength. the alloy currently used is a high strength, low alloy steel known as D6AC, a steel containing about 0.45% carbon, 1% chromium, 1% molybdenum, 0.5% nickel in addition to iron and impurities. Depending upon tempering treatment it is understood to have a K.sub.IC value on the order of 75 Zksi .sqroot.in at a yield strength in the neighborhood of 200,000 psi. The steel is usually or often liquid quenched and this can give rise to dimensional changes. What is desired for such application is a K.sub.IC above 75 ksi .sqroot.in and upwards of a 90-100 Ksi square root inch fracture toughness. But to acheive this level at the sacrifice of strength is not a panacea. Thus, an alloy must also be of high yield strength, i.e., well above 200,000 psi and advantageously at least 220,000 psi.
The commercial steel f '254, known as MS-250, contains about 1.35 to 1.45% titanium together with about 3% molybdenum, 18% nickel and low carbon. It is aged at 900.degree. F. and affords strengths of 240,000-250,000 psi. While strong enough, its K.sub.IC value is somewhat lacking, being around 70 ksi .sqroot.in with Charpy V-Notch value of about 15-20 foot-pounds or slightly higher.